Back to EveryPatent.com
| United States Patent |
6,202,724
|
|
Hiruma
, et al.
|
March 20, 2001
|
Pneumatic tire and method of manufacturing the same
Abstract
A pneumatic tire comprising a silica-rich compound tread provided with an
antistatic conductive rubber layer has improved safty based on durability
and extrusion productivity. In this tire, the silica-rich compound rubber
layer is divided at at least one portion in a width directional of the
tire by a conductive rubber layer extended in a circumferential direction
substantially over the whole thickness of said rubber layer. In this case,
a thickness direction distance a in a width directional section of said
rubber layer and an edge face length b of a divisional rubber layer
consisting of said conductive rubber layer in a width directional section
satisfy a relation formulated by the following expression, a<b.
| Inventors:
|
Hiruma; Masato (Inagi, JP);
Kusano; Akio (Kodaira, JP);
Fujita; Yasuyo (Kodaira, JP)
|
| Assignee:
|
Bridgestone Corporation (Tokyo, JP)
|
| Appl. No.:
|
130879 |
| Filed:
|
August 7, 1998 |
Foreign Application Priority Data
| Aug 07, 1997[JP] | 9-212955 |
| Sep 17, 1997[JP] | 9-251771 |
| Current U.S. Class: |
152/152.1; 152/209.5; 152/209.8; 152/DIG.2; 156/123; 156/128.1; 156/128.6; 156/129 |
| Intern'l Class: |
B60C 001/00; B60C 019/08; B60C 011/00; B29D 030/52 |
| Field of Search: |
152/152.1,209.5,209.8,DIG. 2
156/123,128.1,128.6,129
|
References Cited
U.S. Patent Documents
| 2339546 | Jan., 1944 | Hanson | 152/DIG.
|
| 3931844 | Jan., 1976 | Mirtain | 152/209.
|
| 4385653 | May., 1983 | Okazaki et al. | 152/209.
|
| 4478266 | Oct., 1984 | Pierson et al. | 152/209.
|
| 5225011 | Jul., 1993 | Takino et al. | 152/209.
|
| 5518055 | May., 1996 | Teeple et al. | 152/152.
|
| 5898047 | Apr., 1999 | Howald et al. | 152/152.
|
| 5937926 | Aug., 1999 | Powell | 152/152.
|
| 5942069 | Aug., 1999 | Gerresheim et al. | 152/152.
|
| Foreign Patent Documents |
| 597008 | Mar., 1978 | CH | 152/152.
|
| 341187 | Nov., 1989 | EP | 152/209.
|
| 0 658 452 A1 | Jun., 1995 | EP.
| |
| 718126 | Jun., 1996 | EP | 152/DIG.
|
| 732229 | Sep., 1996 | EP | 152/209.
|
| 747243 | Dec., 1996 | EP | 152/DIG.
|
| 798142 | Oct., 1997 | EP.
| |
| 819555 A1 | Jan., 1998 | EP.
| |
| 819741 A2 | Jan., 1998 | EP.
| |
| 838353 | Apr., 1998 | EP.
| |
| 819741 A3 | Oct., 1998 | EP.
| |
| 544757 | Apr., 1942 | GB | 152/152.
|
| 2-162104 | Jun., 1990 | JP | 152/209.
|
| 4-95508 | Mar., 1992 | JP | 152/209.
|
| 4-133802 | May., 1992 | JP | 152/209.
|
| 6-16013 | Jan., 1994 | JP | 152/209.
|
Other References
Patent Abstracts of Japan, V1999, N10, Aug. 31, 1999 (JP 11-139107, May 25,
1999).
Patent Abstracts of Japan, V1999, N5, May 31, 1999 (JP 11-048711, Feb. 23,
1999).
|
Primary Examiner: Maki; Steven D.
Attorney, Agent or Firm: Sughrue, Mion, Zinn, Macpeak & Seas, PLLC
Claims
What is claimed is:
1. A method of manufacturing a pneumatic tire, the improvement which
comprises the steps of; simultaneously extruding a tread portion of
silica-rich compound and an antistatic conductive rubber layer, and
forming said conductive rubber layer of 0.1-3.0 mm thick to a V- or a
reverse V-shape in a width directional section of the tire at the tread.
2. A method of manufacturing a pneumatic tire, the improvement which
comprises the steps of; dividing a tread portion of silica-rich compound
into three portions in a V- or a reverse V-shape in a width directional
section of the tire, adhering a conductive rubber layer consisting of a
conductive rubber sheet or conductive cement layer to at least one of the
surfaces created from the division, and thereafter connecting the three
tread portions which were divided into one through said conductive rubber
layer to thereby provide the tread portion with a conductive rubber layer
having a V- or a reverse V-shape in the width directional section of the
tire.
3. A method of manufacturing a pneumatic tire as claimed in claim 2,
wherein a temperature is not less than room temperature when both the
divided tread pieces are connected with each other.
4. A method of manufacturing a pneumatic tire as claimed in claim 2,
wherein a press bonding step is provided after connecting said tread
pieces each other.
5. A pneumatic tire comprising; a tread, a silica-rich compound rubber
layer on at least the surface portion of said tread, said silica-rich
compound rubber layer being divided at at least one portion in a width
directional section of the tire by a conductive rubber layer extending in
a circumferential direction substantially over the whole thickness of said
rubber layer, wherein said conductive rubber layer is formed into a V- or
reverse V-shape having a width in the range of 0.1-3 mm in a width
directional section of said tire and a thickness direction distance (a) in
a width directional section of said rubber layer and an edge face length
(b) of said conductive rubber layer in a width directional section
satisfy, a<b, wherein said V-shape conductive rubber layer has upper ends
of said conductive rubber layer at a tread ground surface portion and a
lower end thereof extends to a radially inner position substantially
dividing the tread in a widthwise manner or said reverse V-shape has an
upper end of said conductive layer extending to said tread surface to
substantially divide the tread in a widthwise manner and lower ends
thereof extend to a tread base.
6. A pneumatic tire as claimed in claim 1, wherein a specific resistance
value of said silica-rich compound rubber layer is not less than 10.sup.8
.OMEGA..multidot.cm, and a specific resistance value of said conductive
rubber layer is not more than 10.sup.6 .OMEGA..multidot.cm.
7. A pneumatic tire as claimed in claim 5, wherein said conductive rubber
layer is 0.5-2.0 mm in width.
8. A pneumatic tire as claimed in claim 5, wherein said tread has at least
two-layer structure comprising a cap rubber layer of said silica-rich
compound rubber layer in its surface portion and a base rubber layer which
is in downward from the cap rubber layer in the radial direction.
9. A pneumatic tire as claimed in claim 5, wherein said conductive rubber
layer consists of a rubber cement layer having a specific resistance value
of not more than 10.sup.6 .OMEGA..multidot.cm.
10. A pneumatic tire as claimed in claim 5, wherein said conductive rubber
layer consists of a conductive sheet having a specific resistance value of
not more than 10.sup.6 .OMEGA..multidot.cm.
11. A pneumatic tire as claimed in claim 5, wherein the ratio (A/B) of a
sectional area A of said conductive rubber layer to a sectional area B of
said silica-rich compound rubber layer in a width directional section of
the tire is within a range of 0.02-0.1.
12. A pneumatic tire as claimed in claim 5, wherein said tread has at least
two-layer structure comprising a cap rubber layer of said silica-rich
compound rubber layer in its surface portion and a base rubber layer which
is in downward from the cap rubber layer in the radial direction, and the
ratio (A/B) of said sectional area A of said conductive rubber layer to
said sectional area B of said cap rubber layer in a width directional
section of the tire is within a range of 0.02-0.1.
13. A pneumatic tire as claimed in claim 5, wherein both wings of said
tread are provided with miniside rubber layers.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a pneumatic tire comprising a silica-rich
compound tread rubber provided with an antistatic conductive rubber layer.
It particularly relates to a pneumatic tire having improved safety based
on durability and extrusion productivity.
2. Description of the Related Art
In a prior pneumatic tire, a proper amount of carbon black is contained in
tread rubber, and there is no problem relating to electrical resistance of
a tire or a problem relating to accumulation of charge amount. However,
environmental concerns have recently been serious, and a movement of
lowering fuel consumption of vehicles has been accelerated. In order to
attain low fuel consumption, i.e., mitigation of rolling resistance by
improving tread rubber, it is necessary to decrease carbon black as a
cause for producing hysteresis loss, and there is today given attention to
silica --containing tread rubber by decreasing an amount of carbon black,
as tread rubber having excellent low fuel consumption performance. In
order to coexist running performance and low fuel consumption performance
of a tire at a high level, particularly in a pneumatic radial tire having
cap/base structure of a tread, there is a tendency to increase cases of
using silica-rich compound rubber as rubber for a cap layer. As a result,
there is newly caused a problem relating to electrical resistance and a
problem relating to accumulation of a charge amount.
As a solution of such problems, for example, as disclosed in European
Patent Specification No. 658 452, there has been known a technique of
inserting a conductive rubber layer 20 from a tread surface to tread
underlayer rubber at the width directional center portion of a tread 10
(FIG. 7). In this pneumatic tire, as illustrated, a connected face between
the conductive rubber layer 20 and the silica-rich rubber layer 10
provided in the tread is perpendicular to the ground-contact surface of
the tire.
As described above, a pneumatic tire in which the conductive rubber layer
is provided to be perpendicular to the ground-contact surface of the tire
sufficiently exhibits antistatic effect because the layer is constantly
grounded, but the conductive rubber layer and the silica-rich rubber layer
tend to peel off at the connected face due to strain produced in the tire
when running, and thus there is a problem from the aspect of tire safety.
That is, as disclosed in the above European Patent Specification No. 658
452, if the conductive rubber layer is inserted from the surface to the
underlayer rubber in the silica-rich compound tread, in such a case that
the tire receives the transverse directional force particularly when the
vehicle receives a side wind during high speed running, as shown in FIG.
8, the conductive rubber layer 20, which is heterogeneous from the other
tread portion 10, peels off from the boundary face, and this peeling-off
has a problem to be continuously extended on the circumference of the
tire. Such problem also occurs at the time of revolving and running in
like manner. Once the peeling-off occurs, tire durability is considerably
lowered.
SUMMARY OF THE INVENTION
Therefore, an object of the invention is to provide a pneumatic tire
capable of positively securing antistatic effect without damaging driving
safety based on durability of the tire.
Moreover, another object of the invention is to provide a pneumatic tire
sufficiently exhibiting antistatic effect and simultaneously showing
driving safety viewed from durability, and further having improved
extrusion productivity.
As a result of study for solving the above problems, in case of forming a
current path by applying a conductive rubber layer to a tread, the
inventors have found that the conductive rubber layer can be prevented
from peeling off by extending a divisional layer consisting one conductive
rubber layer to the oblique direction against the thickness direction in a
width directional section of the tread rubber layer, to effectively
disperse force received from the tire transverse direction, and to
restrain movement of the conductive rubber layer and rubber deformation at
the boundary face.
Moreover, the inventors have found that further higher extrusion
productivity can be obtained by arranging a conductive rubber layer of
specific thickness in V-shape or reverse V-shape in a width directional
section at the tread of a silica-rich rubber layer of the tire to form a
conductive path.
According to the invention, there is the provision of a pneumatic tire
having a silica-rich compound rubber layer on at least the surface portion
of a tread, in which the silica-rich compound rubber layer is divided at
at least one portion in a width directional section of the tire by a
conductive rubber layer extended in a circumferential direction
substantially over the whole thickness of the rubber layer, wherein a
thickness direction distance a in a width directional section of the
rubber layer and an edge face length b of a divisional rubber layer
consisting of the conductive rubber layer in a width directional section
satisfy a relation formulated by the following expression, a<b.
In a preferred embodinent of the invention, a specific resistance value of
the silica-rich compound rubber layer after curing is not less than
10.sup.8 .OMEGA..multidot.cm, a specific resistance value of the
conductive rubber layer is not more than 10.sup.6 .OMEGA..multidot.cm, the
conductive rubber layer is 0.1-3.0 mm, preferably 0.5-2.0 mm in width, and
the edge face length b is 50-100% of a ground-contact width W of the tire.
In another preferred embodiment of the invention, a divisional form in a
width directional section of the divisional rubber layer shows a
substantially linear shape or a corrugated shape.
In the other preferred embodiment of the invention, the tread has at least
two-layer structure comprising a cap rubber layer of the silica-rich
compound rubber layer in its surface portion and a base rubber layer which
is in downward from the cap rubber layer in the radial direction.
According to the invention, it is preferable that the conductive rubber
layer consists of a rubber cement layer or conductive sheet having a
specific resistance value of not more than 10.sup.6 .OMEGA..multidot.cm
after curing.
In the still other preferred embodiment of the invention, the conductive
rubber layer is formed into a V- or a reverse V-shape with 0.1-3.0 mm
thick in a width directional section of the tire. In this case, it is
favorable that the ratio (A/B) of a sectional area A of the conductive
rubber layer to a sectional area B of the silica-rich compound rubber
layer in a width directional section of the tire is within a range of
0.02-0.1, and when the tread has at least two-layer structure comprising a
cap rubber layer of the silica-rich compound rubber layer in its surface
portion and a base rubber layer which is downward from the cap rubber
layer in the radial direction, the ratio (A/B) of the sectional area A to
the sectional area B of the cap rubber layer is within a range of
0.02-0.1.
According to the invention, both wings of the tread may be provided with
miniside rubber layers.
Further, the invention provides a method of manufacturing a pneumatic tire,
which comprises simultaneously extruding a tread portion of silica-rich
compound and an antistatic conductive rubber layer, so as to form the
conductive rubber layer of 0.1-3.0 mm thick into a V- or a reverse V-shape
in a width directional section of the tire at the tread.
Furthermore, the invention provides a method of manufacturing a pneumatic
tire, which comprises dividing a tread portion of silica rich compound
into three portions in a V- or a reverse V-shape in a width directional
section of the tire, adhering a conductive rubber layer consisting of a
conductive rubber sheet or conductive cement layer to at least one of the
surfaces resulted from the division, thereafter connecting both the
divided tread pieces into one through the conductive rubber layer.
In the latter method invention, it is preferable that the temperature is
not less than room temperature when both the divided tread pieces are
connected with each other, and a press bonding step is provided after
connecting the tread pieces each other.
In the pneumatic tire according to the invention, the conductive rubber
layer arranged in a silica-rich compound tread is oblique against a tread
thickness direction in a width directional section of the tire,
particularly formed into a V-shape or a reverse V-shape, to widen a
connecting area between the conductive rubber layer and the silica-rich
rubber layer, to disperse stress due to strain applied to the tire during
running, that is, to minimize stress applied to any point of the
connecting face, and as a result, anti-peelability at such connecting face
is improved. Moreover, in case of forming a silica-rich tread of the
pneumatic tire according to the invention by extrusion, the volume balance
of the conductive rubber layer with the silica-rich rubber extruded
simultaneously therewith is kept, as compared with prior art, particularly
the volume ratio can be optimized, thereby enhancing extrusion speed and
improving productivity.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional perspective view schematically showing the tread
portion of a pneumatic tire as one example of the invention.
FIG. 2 is a sectional perspective view schematically showing the tread
portion of a pneumatic tire as another example of the invention.
FIG. 3 is a sectional perspective view schematically showing the tread
portion of a pneumatic tire as a further example of the invention.
FIG. 4 is a sectional perspective view schematically showing the tread
portion of a pneumatic tire as a still further example of the invention.
FIG. 5 is a sectional perspective view schematically showing the tread
portion of a pneumatic tire as a still more example of the invention.
FIG. 6 is a sectional perspective view schematically showing the tread
portion of a pneumatic tire of Conventional Example 1.
FIG. 7 is a sectional perspective view schematically showing the tread
portion of a pneumatic tire used in Comparative Example.
FIG. 8 is a sectional perspective view schematically showing the state of
peeling occurrence at the tread portion of a pneumatic tire used in
Comparative Example.
FIG. 9 is a sectional view schematically showing the tread portion of a
pneumatic tire of Conventional Example 2.
FIG. 10 is a schematic diagram of a specific resistance value measuring
device used in Examples.
FIG. 11(a) and FIG. 11(b) are an explanatory view showing the measuring
method of a specific resistance value.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Diene rubber used for a rubber composition for a conductive rubber layer
capable of materializing specific resistance of not more than 10.sup.6
.OMEGA..multidot.cm according to the present invention preferably includes
at least one of styrene butadiene rubber (SBR), butadiene rubber (BR) or
natural rubber (NR) viewed from durability.
Moreover, in the rubber composition for the conductive rubber layer, it is
preferable to use carbon black having not less than 130 m.sup.2 /g of a
nitrogen adsoprtion specific surface area (N.sub.2 SA) and not less than
110 ml/100 g of a dibutyl phthalate oil absorption amount (DBP). The use
of such carbon black of small particle size and high structure improves
durability of a rubber layer having a current path and exhibits antistatic
effect till the running end of a tire. Here, N.sub.2 SA is a value
obtained according to ASTM D3037-89, and DBP is a value obtained according
to ASTM D2411-90, respectively.
If a compounding amount of such carbon black is not more than 40 parts by
weight based on 100 parts by weight of diene rubber, reinforcement is not
sufficient. If it exceeds 100 parts by weight, when a softener is not
sufficient, the layer becomes too hard after curing and produces cracks.
When the softener is too great, abrasion resistance is lowered. Moreover,
as compounding agents other than carbon black, use may be made of
compounding agents, usually used in rubber products, such as curing agent,
vulcanization accelerator, vulcanization acceleration assistant, softener,
antioxidant and the like in usual amount.
According to the invention, the conductive rubber layer consists preferably
of a rubber cement layer or conductive sheet having a specific resistance
value of not more than 10.sup.6 .OMEGA..multidot.cm after curing. Here, it
is possible to use water as a solvent for the rubber cement layer, but it
is preferable to use an organic solvent as a base for stabilizing the
quality. As organic solvents, mention may be made of hexane, petroleum
ether, heptane, tetrahydrofuran (THF), cyclohexane and the like,
preferably hexane. The conductive rubber layer consisting of a rubber
cement layer can be, for example, provided by pouring into corrugated cuts
of a tread or applying from the surface to the base of a tread. Such cuts
can be formed by cutting means such as a cutter and the like. Rubber
cement has such an advantage that it is easily introduced into cuts. On
the other hand, the conductive sheet can be provided by inserting into
cuts of the tread at the time of molding a tire.
Next, the structure of a pneumatic tire as one embodiment of the invention
is explained in detail.
In the present preferable pneumatic tire, as shown in FIG. 1, a conductive
rubber layer 2 is extended in a silica-rich tire tread 1 from a tread
surface at at least one potion of its width direction substantially over
the whole thickness a of the tread 1 in the circumferential direction, and
the tread 1 is divided by the conductive rubber layer in a width
directional section. In the present invention, it is important that an
edge face length b of a divisional rubber layer consisting of the
conductive rubber layer 2 of 0.1-3.0 mm, preferably 0.5-2.0 mm in width is
longer than the whole thickness a. In the present preferable embodiment
shown in FIG. 1, the divisional form in a width directional section of the
divisional rubber layer is linearly extended in the oblique direction
against the direction of the thickness a.
If a width of the conductive rubber layer 2 is not less than 0.1 mm, the
introduction of the conductive rubber layer in the form of cement is not
insufficient, and the conductive rubber layer is positively introduced
over the whole width of a tread. Moreover, if the width is not more than
3.0 mm, the rolling resistance of a tire is not worsened, and the boundary
face between the tread rubber and the conductive rubber layer is not
affected by peeling off. Moreover, as the divisional face formed by the
conductive rubber layer 2 is extended in the oblique direction against the
thickness direction in a width directional section of the tread rubber
layer, force applied to a transverse direction of the tire is effectively
dispersed, movement of the conductive rubber layer and rubber deformation
of the boundary face between the conductive rubber layer and the tread
rubber are restrained, and as a result the conductive rubber layer can be
prevented from peeling off.
The edge face length b in a width directional section of the divisional
rubber layer consisting of the conductive rubber layer 2 is preferably
50-100% of a ground-contact width W of the tire. If the length is less
than 50%, force dispersion in the tread thickness direction is
insufficient, and thus improvement of durability cannot be expected. If
the length exceeds 100%, the conductive layer deviates from the
ground-contact surface and thus antistatic effect cannot be obtained.
Here, "ground-contact width" means the maximum linear distance in the tire
axial direction at the tire ground-contact surface when prescribed weight
is added by mounting the tire on a rim to be applied, setting prescribed
atmospheric pressure, and placing the tire perpendicuclar to a flat board
at the standstill, and in JATMA standard, according to 1996 JATMA YEAR
BOOK, the tire is mounted on a standard rim based on the maximum load
capacity at applicable size and ply rating and atmospheric pressure
(maximum atmospheric pressure) corresponding thereto. According to a use
place or a manufacturing place, TRA standard and ETRTO standard are kept,
respectively, when they are applied to.
Moreover, the divisional form in a width directional section of the
divisional face formed by the conductive rubber layer 2 is preferably
substantially linear as shown in FIG. 1, but may preferably be corrugated
as shown in FIG. 2.
Furthermore, silica-rich tread rubber is compounded with a large amount of
silica for coexsisting running performance on wet road surface and low
fuel consumption performance of a tire at a high level. Therefore, a
specific resistance value of the tire tread 1 is not less than 10.sup.8
.OMEGA..multidot.cm.
According to the invention, as shown in FIG. 3, even if the tread has a
two-layer structure provided with a cap rubber layer 3 of the silica-rich
compound rubber on its surface portion and a base rubber layer 4 which is
in downward from the cap rubber layer in the radial direction, and the
conductive rubber layer 2 is substantially extended over the whole
thickness of the cap rubber layer 3 in the circumferential direction, the
same effect of the invention can be obtained as described above.
Next, the structure of a pneumatic tire as another embodiment of the
invention is explained in detail.
In a preferable embodiment of the pneumatic tire of the invention shown in
FIG. 4, the tread 1 consists of a silica-rich rubber layer, and both wings
of the tread 1 are provided with miniside rubber layers 5. A specific
resistance value of this tread 1 is not less than 10.sup.8
.OMEGA..multidot.cm due to silica-rich compound. In this tread 1 is formed
a V-shaped conductive rubber layer 2 in a width directional section of the
tire, both the tread upper ends of the conductive rubber layer 2 are
within the ground-contact surface, and the tread lower end is extended to
a position substantially dividing the tread 1 in a width directional
section of the tire. Moreover, here, "substantially divide" includes the
case when discharge effect can be obtained in the same manner as the case
of complete division, and thinness is remained to a degree of serving for
connection at the time of molding.
When connecting areas of the conductive rubber layer and the silica-rich
rubber layer in the tread of the present pneumatic tire and in the prior
silica rich tread of a tire (FIG. 9) of the same kind having a conductive
rubber layer are regarded as S.sub.B and S.sub.A, respectively, S.sub.A
<S.sub.B, and the peeling-off of the former is more difficult. Moreover,
since the conductive rubber layer 2 divides the tread 1 into three
portions at point p in FIG. 4, it is possible to favorably secure
conductivity of the tread. Furthermore, in case of forming a silica-rich
tread 1 by extrusion, the volumes of the conductive rubber layer and the
other rubber layers are well balanced as compared with the prior tread
shown in FIG. 9, so that extrusion speeds of each layer can be made same,
and thus workability is excellent.
In the aforementioned case, a thickness of the conductive rubber layer 2 is
preferably 0.1-3.0 mm. If this width is not less than 0.1 mm, it is
sufficient for forming a current path. If the width is not more than 3 mm,
the rolling resistance of the tire is not worsened.
Moreover, the ratio (A/B) of the sectional area A of the conductive rubber
layer 2 as a conductive rubber sheet to the sectional area B of the tread
1 in a width directional section of the tire is preferably within a range
of 0.02-0.1. If this ratio is less than 0.02, any improved effect of
extrusion productivity cannot be expected, while if the ratio exceeds 0.1,
it is difficult to coexist running performance and low fuel consumption
performance of a tire as characteristics of silica compound tread at a
high level.
Furthermore, even in case of providing the conductive rubber layer 2 in a
V-shape as described above, the tread 1 may have at least two-layer
structure comprising a cap rubber layer of the silica-rich compound in its
surface portion and a base rubber layer which is in downward from the cap
rubber layer in the radial direction. In this case, the ratio (A/B) of the
sectional area A of the conductive rubber sheet to the sectional area B of
the cap rubber layer in a tire width directional section of the tire is
preferably within a range of 0.02-0.1
A further preferred embodiment of the present pneumatic tire is shown in
FIG. 5. In FIG. 5, a conductive rubber layer 2 is formed in a reverse
V-shape in a width directional section of the tire at the same tread 1.
The tread upper end of the conductive rubber layer 2 is extended to a
position substantially dividing the tread 1 in a width directional section
of the tire, and both the tread lower ends reach the tread base. Here,
"substantially divide" means the same as described above.
Next, a method of manufacturing a pneumatic tire according to the
invention, in which a conductive rubber layer is formed into a V- or a
reverse V-shape in a width directional section of the tire is explained in
detail.
In a preferred embodiment of the manufacturing method of the invention, the
simultaneous extrusion of a tread portion of silica-rich compound and an
antistatic conductive rubber layer forms the conductive rubber layer into
a V- or a reverse V-shape in thickness of 0.1-3.0 mm in a width
directional section of the tire at the tread. Such simultaneous extrusion
can balance the volume of the conductive rubber layer with that of the
simultaneously extruded silica-rich rubber since the conductive rubber
layer is formed into a V- or a reverse V-shape in a width directional
section of the tire. As a result, an extrusion speed can be increased by
optimizing the volume ratio, and thus productivity can be improved as
compared with the prior art.
Moreover, another preferred embodiment of the invention, first, a
silica-rich tread portion is divided into three parts in a V- or a reverse
V-shape in a width directional section of the tire. Such division is
carried out by extruding three parts separately, or by extruding one tread
and then dividing (slitting) it into three parts by means of a cutter and
the like. Then, the conductive rubber layer consisting of a conductive
rubber sheet or conductive cement layer is adhered to at least one of the
surfaces resulted from the division. After adhering the conductive rubber
layer to the resulting surface, both the divided tread pieces are
connected into one through the conductive rubber layer at a temperature of
preferably not less than room temperature. In case of connecting and press
bonding through the conductive rubber layer, the connecting portion is
press-bonded by passing through a stitcher roll or a profile roll.
As explained above, according to the present pneumatic tire, in which a
current path is formed by applying the conductive rubber layer to a tread,
the divisional face formed by one conductive rubber layer is extended in
the oblique direction against the thickness direction in a width
directional section of a tread rubber layer, whereby force applied to a
transverse direction of the tire is effectively dispersed to prevent the
conductive rubber layer from peeling off, and antistatic effect can
positively be secured without damaging durability. Moreover, the formation
of a conductive path by arranging the V- or the reverse V-shaped
conductive rubber layer in a width directional section of the tire at the
tread of the silica-rich rubber layer can sufficiently exhibit antistatic
effect and at the same time, can restrain crack occurrence at the
connecting face between the conductive rubber layer and the silica-rich
rubber layer, and as a result, the present pneumatic tire can be provided
with excellent safety and high extrusion productivity.
The following examples are given for the purpose of illustration of this
invention and are not intended as limitations thereof.
EXAMPLES 1-2
Comparative Example and Conventional Example
According to compounding prescriptions shown in the following Tables 1 and
2, rubber compositions used for a silica-rich rubber layer and a
conductive rubber layer of a pneumatic radial tire were prepared,
respectively.
TABLE 1
Silica-rich rubber layer
Amount
Styrene butadiene rubber .sup.*1 96 (part by weight)
Butadiene rubber .sup.*2 30
SiO.sub.2 .sup.*3 60
Carbon black (N234) .sup.*4 20
Silane coupling agent .sup.*5 6
ZnO 3
Stearic acid 2
Aromatic oil 10
Vulcanization accelerator (CBS) .sup.*6 1.5
Vulcanization accelerator (DPG) .sup.*7 2
Sulfur 1.5
.sup.*1 SBR1712 made by JSR
.sup.*2 96% cis-bound
.sup.*3 Nipsil VN3
.sup.*4 N.sub.2 SA: 126m.sup.2 /g DBP: 125 ml/100 g
.sup.*5 Si69 made by DEGUSSA
.sup.*6 N-cyclohexyl-2-benzothiazylsulfenamide
.sup.*7 diphenylguanidine
TABLE 2
Conductive rubber layer
Amount
Natural rubber 40 (part by weight)
Styrene butadiene rubber .sup.*8 60
Carbon black (N134) .sup.*9 60
Aromatic oil 15
ZnO 2
Antioxidant .sup.*10 1
Vulcanization accelerator (DPG) 0.2
Vulcanization accelerator (NS) .sup.*11 0.8
Sulfur 1.5
.sup.*8 SBR1500 made by JSR
.sup.*9 N.sub.2 SA: 146 m.sup.2 /g DBP: 127 ml/100 g
.sup.*10 N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine
.sup.*11 N-tert-butyl-2-benzothiazolylsulfenamide
The thus obtained conductive rubbers were applied as a conductive rubber
layer 2 to a tread 1 as shown in FIGS. 1, 3 and 7, and test pneumatic
radial tires of size 185/65R14 were manufactured.
In Example 1, as shown in FIG. 1, the divisional face formed by one
conductive rubber layer 2 is linearly extended in the oblique direction
against the thickness direction in a width directional section of a tread
1. The thickness of this conductive rubber layer 2 in a width directional
section of the tire is 1 mm, and the edge face length b in width
directional section of the divisional rubber layer consisting of the
conductive rubber layer 2 is 80% of the tire ground-contact width W.
In Example 2, as shown in FIG. 3, in the tread of cap/base structure, the
divisional face formed by one conductive rubber layer 2 is linearly
extended in the oblique direction against the thickness direction in a
tread width directional section of the tire, and reached the under surface
of the base layer. The thickness of this conductive rubber layer 2 in a
width directional section of the tire is 1 mm, and the edge face length b
in a width directional section of the divisional rubber layer consisting
of the conductive rubber layer 2 is 80 % of a ground-contact width W of
the tire.
Conventional Example 1 is, as shown in FIG. 6, an example of the same tire
as the above examples, except that the conductive rubber layer 2 is not
inserted.
Comparative Example is, as shown in FIG. 7, an example of the same tire as
the above examples, except that a conductive rubber layer of 5 mm
thickness in a width directional section of the tire is continuously
formed in the circumferential direction at the tire center.
Resistance values (electrical resistance values) of these tires are
obtained as follows.
That is, resistance values were measured as shown in FIG. 10 with the use
of a high resistance meter of Model HP4339A made by HEWLETT PACKARD
according to WdK 110 sheet 3 of GERMAN ASSOCIATION OF RUBBER INDUSTRY. In
FIG. 10, 11 is a tire, 12 a steel plate, 13 an insulator, 14 a high
resistance meter, and measurement was carried out by flowing a current of
1000V between the steel plate 12 on the insulator 13 and a rim of the tire
11.
Moreover, a specific resistance values of the conductive rubber layer 2
were obtained as follows.
That is, a disk-shaped sample was manufactured, an electrical resistance
value R at the portion of radius: r=2.5 cm and thickness t=0.2 cm was
measured with the use of an insulation resistance test box made by
ADVANCE, and a specific resistance value .rho. was calculated by the
following equation
.rho.=(a/t)R.
In the equation, a is a sectional area (=.pi..times.r.sup.2), and t is a
thickness. In FIG. 11, A shows a main electrode, B a counter electrode, C
a guard electrode and t a thickness of the sample.
A test tire was mounted on the actual vehicle, which was circularly
revolved under the condition of lateral G of 0.4-0.5 at rudius of 80 m,
and then peeling-off at the boundary face between the tread rubber and the
conductive rubber layer was checked after 20 rounds. The case of
apparently producing peeling-off was made durability x, and the case of no
peeling-off was made durability .smallcircle.. The thus results obtained
are shown in the following Table 3.
TABLE 3
Conven- Compar-
tional ative Exam-
Example 1 Example Example 1 ple 2
Conduc- Inserting Fig. 6 Fig. 7 Fig. 1 Fig. 3
tive position
rubber (refer to
layer drawing)
Specific -- 10.sup.5 10.sup.5 10.sup.5
resistance value
(.OMEGA. .multidot. cm)
Tire width -- 5 1 1
directional
thickness (mm)
Isolation length -- -- 80 80
(b/W) (%)
Electrical resistance (.OMEGA.) 10.sup.11 10.sup.6 10.sup.6 10.sup.6
Durability .largecircle. X .largecircle.
.largecircle.
EXAMPLE 3
With the use of the resulting rubber compositions for the silica-rich tread
and the conductive rubber layer, a pneumatic radial tire (size 185/60R14)
of tread structure shown in FIG. 4 was manufactured. This tire has a value
of 0.024 in the ratio (A/B) of the sectional area A of the conductive
rubber layer 2 to the sectional area B of the silica-rich tread rubber 1
in a width directional section of the tire. Moreover, the conductive
rubber layer is 1.0 mm in thickness.
Conventional Example 2
With the use of the same rubber compositions as in Example 3, respectively,
a pneumatic radial tire of the same size with the tread structure shown in
FIG. 9 was manufactured. This tire has a value of 0.012 in the ratio (A/B)
of the sectional area A of the conductive rubber layer 2 to the sectional
area B of the silica-rich tread rubber 1 in a width directional section of
the tire. Moreover, the width directional thickness of the conductive
rubber layer is 1.0 mm.
A tread extrusion speed of Example 3 is indicated by an index based on 100
of Conventional Example 2. The larger the numerical value, the better the
productivity. Moreover, tires of Example and Conventional Example were
mounted on actual vehicles, a 2-hour slalom running test was carried out
at a speed of 60 km/hr, occurrence of cracks on the connecting face
between the conductive rubber layer and the silica-rich rubber layer at
the tread was examined. The result obtained is shown in the following
Table 4.
TABLE 4
Conventional Example 2 Example 3
Cross sectional ratio 0.012 0.024
(A/B)
Productivity 100 200
(extrusion speed) (index)
Existence of crack Present Non
occurrence
Top